Add Spark engines

[hsbadr]

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Add support for Spark ML clustering using sparklyr::ml_kmeans() , sparklyr::ml_bisecting_kmeans(), and sparklyr::ml_gaussian_mixture() + sparklyr::ml_predict() for predictions.

[EmilHvitfeldt]

Thank you @hsbadr 👋 I added a feature label, and we will look into this once we get back